The use of polybrominated polystyrenes as flame retardant additives for polyolefin-based molding materials is described in U.S. Pat. No. 3,474,067. That patent describes combinations of molding materials based on polyethylene and polypropylene with several different nuclear-brominated polystyrenes together with synergists such as antimony trioxide. The use of poly-(tribromostyrene) was particularly emphasized, as in Table 1 of the patent. However, the patent does not disclose the molecular weight of the brominated polystyrene, nor how it was produced.
U.S. Pat. No. 3,973,354 describes a flame-resistant thermoplastic glass-fiber reinforced polyester molding composition, containing a saturated polyester, a synergist, and from 3% to 30% by weight of the composition of poly(2,4,6-tribromostyrene). The patent reported that the poly(2,4,6-tribromostyrene) was a commercially available product with a density of 2.3 grams/cm.sup.3 and a bromine content of 69%. The process for making the product is not described in this patent.
The direct nuclear halogenation of polystyrene in solution, in the presence of iron chloride or aluminum chloride, with elemental chlorine, is described in British Pat. No. 364,873.
The direct bromination of polystyrene is described in U.S. Pat. No. 3,050,476. A suspension of polystyrene particles is heated in the presence of bromine, to cause bromine to combine chemically with the polymer particles. Bromine is added to a very low level of bromination.
U.S. Pat. No. 3,845,146, describes the bromination of aromatic compounds such as lower alkyl benzenes, utilizing bromine chloride as the brominating agent, with a catalyst such as aluminum chloride. The reaction is conducted in a closed reaction vessel under autogenous pressure, often in the range from about 50 psig to about 100 psig.
Cubbon and Smith describe the synthesis and polymerization of tribromostyrene in an article in Polymer, 10, 479-487 (1969). Tribromostyrene is prepared in a multiple step reaction, by first effecting the addition of hydrogen bromide to the double bond of styrene to produce 2-bromoethylbenzene, then reacting that material with elemental bromine in the presence of iron chloride, to introduce bromine into the nucleus. Hydrogen bromide is then removed, to re-introduce the double bond, by reaction with potassium ethoxide, at about 30.degree. C. The product was identified through its nuclear magnetic resonance spectrum as 2,4,5-tribromostyrene. The rate of polymerization of this tribrominated styrene was observed in benzene solution at 30.degree. C. Upon comparing its rate of polymerization with that of dibromostyrene, the conclusion was reached that the introduction of bromine atoms activates the vinyl group toward polymerization, with the tribromostyrene polymerizing at a more rapid rate than the dibromostyrene, which in turn polymerizes at a more rapid rate than styrene.
Other trihalo aromatic compounds are well known. For example, the six isomers of tribromotoluene were characterized by Cohen and Dutt in Journal of the Chemical Society, 105, 501-521 (1914). U.S. Pat. No. 2,483,753, describes trichlorostyrene, and a method for producing it is described in the Journal of General Chemistry USSR, 28, 3303-3306 (1958). U.S. Pat. No. 2,398,736 teaches that dichlorostyrene and dibromostyrene are equivalent to each other for copolymerization with styrene. Polytrichlorostyrenes are also described, along with processes for producing them, in U.S. Pat. Nos. 2,406,319 and 2,463,897.
In German Pat. No. 1,570,395, Example 3 purports to describe the production of poly-(2,4,6-tribromostyrene), and Example 4 purports to describe the production of, simply, poly-(tribromostyrene).
More recently several patents have issued that describe the production and fire retardant use of brominated polystyrene oligomers. These oligomers may be prepared by the action of elemental bromine on the hydrogenated polystyrene oligomer, as in the Naarmann et al. U.S. Pat. Nos. 4,074,033 and 4,143,221, where the catalyst used was aluminum chloride (a Lewis acid catalyst), or alternatively, by the polymerization of brominated styrene.
In Wurmb et al., U.S. Pat. No. 4,107,231, such brominated oligomers are described as useful in imparting flame retardant properties to linear polyesters. The degree of polymerization of the oligomer may be in the range from 3 to 20. The use of a tribrominated oligomer is mentioned.
In Theysohn et al., U.S. Pat. No. 4,137,212, similar brominated polystyrene oligomers, with a degree of polymerization of from 3 to 90, are disclosed as useful for flameproofing molded nylon compositions. The tribrominated oligomer is mentioned.
In Neuberg et al., U.S. Pat. No. 4,151,223, the brominated oligomer may have a degree of polymerization in the range from 3 to about 100, and is described as useful for imparting flame-retardant properties to fibers and filaments of linear thermoplastic polyesters. This patent points out that the halogenated oligomeric styrene may be either chlorinated or brominated, and the degree of halogenation may run the complete spectrum.